Coal-fired power plants are utilized throughout the world to generate electricity. Typically, the coal is either in a pulverized form or within a slurry is combusted to generate heat within a boiler to raise steam. The steam is passed into a steam turbine to generate electrical power. There has been recent interest in capturing carbon dioxide from power plants that use coal and other carbonaceous feed stock such as asphalt, heavy oil, petroleum coke, biomass or natural gas. An integrated gasification and combined cycle (IGCC) is proposed as a preferred method of power generation when carbon dioxide capture is required. In IGCC, gasification of fuel produces a synthesis gas containing mainly hydrogen, carbon monoxide and carbon dioxide with some amount of methane and sulfur and chloride containing impurities. In a typical gasifier the carbonaceous feed is reacted with steam and oxygen to produce the synthesis gas. Typically, the oxygen is provided to the gasifier by a cryogenic air separation unit.
In IGCC, the synthesis gas produced as a result of the gasification is typically cooled to a temperature suitable for its further processing in a water-gas shift reactor to increase the hydrogen and carbon dioxide content of the synthesis gas. The water-gas shift reactor also hydrolyzes most of the carbonyl sulfide into hydrogen sulfide. The synthesis gas is then further cooled for carbon dioxide and hydrogen sulfide separation within a solvent scrubbing plant employing physical or chemical absorption for separation of the carbon dioxide and hydrogen sulfides and carbonyl sulfide from the synthesis gas. This allows for the capture and sequestration of the carbon dioxide which is present within the synthesis gas. The resulting hydrogen-rich gas is then fed to a gas turbine that is coupled to an electrical generator to generate electricity. Heat is recovered from the cooling of the raw synthesis gas stream, from cooling the heated discharge from the water-gas shift reactor, and cooling the exhaust from the gas turbine to raise steam and to generate additional electrical power from a steam turbine.
As can be appreciated, the IGCC is environmentally very advantageous in that a clean burning synthesis gas stream is used to power the gas turbine while at the same time, the carbon dioxide produced by the gasification can be captured for use in other industrial processes, for enhanced oil recovery or for sequestration. The disadvantage of such an IGCC cycle is the high energy penalty associated with the air separation and solvent scrubbing plants. Additionally, the recovery of heat energy in several stages is inherently inefficient in that such heat recovery always involves loss and in any case, the heat is recovered at a low temperature. Lastly, the use of solvent scrubbing plants, water-gas shift reactors and gas turbines is an expensive proposition given their high capital costs.
The use of oxygen transport membrane (OTM) systems have also been contemplated in connection with boilers to generate products used to produce electricity, as disclosed in U.S. Pat. Nos. 6,394,043; 6,382,958; 6,562,104; and, more particularly U.S. Pat. Nos. 7,856,829 and 8,196,387. In such OTM based systems, oxygen is separated from the air with the use of a ceramic membrane that is capable of oxygen ion transport at elevated temperatures. The oxygen ionizes on one surface of the membrane by gaining electrons to form the oxygen ions. Under a driving force of a partial pressure differential, the oxygen ions pass through the membrane and either react with a fuel or recombine to elemental oxygen liberating the electrons used in the ionization of the oxygen.
An alternative oxygen transport membrane based system and the closest prior art for advanced power cycle systems are disclosed in U.S. Pat. Nos. 7,856,829 and 8,196,387. These prior art advanced power cycle systems require the oxygen transport membrane to operate in a high pressure environment of about 350 psig, particularly in the POx stages which directly heat the synthesis gas from the gasifier ahead of any expansion stages. One of the recognized problems associated with oxygen transport membranes is that when operating in severe environments, such as high pressure environments, the reliability of the oxygen transport membranes typically suffer resulting in more membrane failures and associated system operating downtime and maintenance costs. In addition, oxygen transport membranes that are designed to operate in higher pressure environments typically require very thick support layers thus significantly increasing the cost of the oxygen transport membranes and associated reactors.
In lieu of operating the oxygen transport membranes at such high pressures, prior art solutions contemplate regulating a portion of the high pressure gasifier stream to lower pressure levels before introduction to the oxygen transport membranes based reactors. Regulating or reducing the high pressure gasifier stream involves specialized equipment and adversely impacts the overall economics and efficiency of the oxygen transport membrane based power system. Also, the prior art solutions requires the oxygen transport membranes to operate at very low fuel utilization or conversions rates making it difficult to control the oxygen flux across the membranes, since oxygen flux is a decreasing function of the fuel utilization or fuel conversion.
What is needed therefore is a method for generating power from a high pressure gasifier stream using an oxygen transport membrane based system designed to operate at lower pressures and is more cost efficient than the current prior art oxygen transport membrane based power cycle systems and solutions. In particular, what is needed is an oxygen transport membrane based power cycle system that facilitates operation of the oxygen transport membrane at low fuel pressures with high fuel utilization or fuel conversion.